Common medium multichannel exchange system



ec. 22, 1936. D 2,964,961

COMMON MEDIUM MULTICHANNEL EXCHANGE SYSTEM Filed May 26, 1934 2Sheets-Sheet l M zy m INVENTOR 2 Wfifidd ATTORNEY Dec. 22, 1936'. w. H.TIDD COMMON MEDIUM MULTICHANNEL EXCHANGE SYSTEM Filed May 26, 1934 2Sheets-Sheet 2 v I?) make Contact; qfRL Yb SR Zb bawery 6 a1": B CentralRepeating Stan.

b Mm C antral OffLbe INVENTOR WET/add ATTORN EY Patented Dec. 22, 1936UNETED STATES CGMMON MEDIUM MULTICHANNEL EXCHANGE SYSTEM WarrenHitchcock Tidd, White Plains, N. Y., as-

signor to American Telephone and Telegraph Company, a corporation of NewYork Application May 26, 1934, Serial No. 727,798

27 Claims.

This invention has for its object the provision of an exchange system inwhich communication between subscribers is carried out over channels ofdifferent frequency derived from a common transmission medium. Moreparticularly, the invention is designed to utilize as a substitute forthe many subscriber pairs which are employed. in the ordinary telephoneexchange system the many channels which are obtainable in thehigh-frequency art as it is now unfolding and to provide means wherebythese channels may be selected by subscribers at will and employed forinterconnection.

In the telephone exchange systems which have heretofore been provided ithas been the practice to group subscribers in central oiiice areas. Eachsubscriber is connected to his own central oflice by means of anindividual wire circuit or by a party line arranged for non-simultaneoususe by a few subscribers. When a subscriber desires to call anothersubscriber he utilizes his own line to the central ofice, and uponpassing the desired number either to an operator or to an automaticswitching mechanism is connected to another line which extends eitherdirectly or via another oiiice or ofiices to the called party. For asystem of this kind many thousands of subscriber circuits are necessaryin each central office area. These are commonly provided in the form ofa network of multiple pair cables extending over the area.

In accordance with the present invention it is proposed to substitutefor such individual wire circuits extending to each subscriber atransmission medium which is common to all subscribers and which willaccommodate a range of frequencies sufiiciently wide for a large numberof telephone channels. Interconnection between subscribers is thencarried out over these highfrequency channels.

The essence of the invention resides in making available to a largenumber of subscribers a transmission medium capable of accommodating awide range of frequencies so that the subscribers may utilize, for thepurposes of intercommunication, a plurality of signaling channelsderived from the wide-band medium. A transmission medium capable ofhandling the large number of channels required for the exchange systemof the invention may be obtained by utilizing either radio transmissionin space or high-frequency transmission over a conducting or guidingmedium.

Considering, first, the use of radio as the medium for transmitting thewide band of frequencies contemplated by the invention, the invention isdesigned to employ whatever partof the frequency range may be available.Heretofore, however, the radio frequency spectrum has scarcely afiordedsufiicient frequency space to yield the channel capacity required for alocal telephone exchange system. With the opening up of the field ofultra-short waves this limitation as to channel capacity is removed.Thus, for example, between one and two meters there is a total frequencyrange of 150 megacycles while the range between 10 and 20 centimeters is1500 megacycles. The frequency ranges obtainable through the use ofstill shorter waves are much greater.

With these ultra-high frequencies, however, nature imposes anotherlimitation, i. e., the tendency of the waves to take on quasi-opticalcharacteristics, so that they project only in straight lines and do notbend around corners to any considerable extent. Thus, it becomesnecessary that subscribers utilizing such waves have a fairly clear lineof sight between their antennas. Some diiiraction around obstacles may,of course, be obtained, but, in general, reasonably clear line-of-sightprojection without intervening obstacles appears to be called for. This,of course, has the advantage of making it possible to utilize the sameultra-high frequencies in diiferent areas.

The line-oi-sight characteristics of the ultrashort waves must berecognized in the design of a telephone exchange system utilizing thistechnique. In one form of the invention it is proposed to provide in thecenter of a community a repeating station with antennas mountedsufficiently high above the surrounding buildings to make possible thenecessary directness of transmission between these antennas andpractically any point in the territory to be served. One subscribersstation is then placed in connection with another by transmitting firstto the central repeating station and having the repeating station repeatthe transmission to the desired other subscriber located in another partof the community. The central repeating station acts as a means forcatching the waves from one subscriber at a relatively high point andtransmitting them down again to a second subscriber, thus lifting thetransmission path above any obstacles lying between the subscribersantennas.

In order that such a repeating station may be economical, it is proposedto make it capable of handling simultaneously a wide band of frequenciessuch as would accommodate hundreds or perhaps thousands of channels. Theparticular arrangement of the repeating station will be hereinafterdescribed.

As an alternative to the use of a radio transmission medium, theinvention contemplates the employment of a guiding or conductingtransmission path suitable for the required band of frequencies. Such atransmission path would be provided in the form of a network, extendingto all the subscribers within a given area. Conceivably, such a networkmight be composed. of ordinary wire circuits branched and interconnectedso as to make the common circuit available to each subscriber. Thus, thewire network might be somewhat similar to the network employed fordistributing light and power currents, except that a simple two-wirenetwork without multi-phase connections, voltage trans formationarrangements, etc., would suifice. It would, however, be possible tocarry out the invention by employing the light and power network itselfand this is contemplated within the scope of the invention.

The preferred form of guiding transmission medium, however, is one whichis designed for the transmission of a wide band of frequencies withcomparatively low attenuation and preferably also with shielding tominimize the effect of external disturbances. Such a transmission mediummay be found in a circuit consisting of two conductors disposedcoaxially with respect to one another. In a properly designed coaxialcircuit, as will be hereinafter explained, low attenuation at highfrequencies may be attained by the employment of conductors of suitablylow high-frequency resistance and the use of a substantially gaseousdielectric between the conductors. With this type of circuit the outerconductor serves also as a shield whose protective effect becomes morenearly perfect as the frequency is increased, so that at high frequencies practically complete immunity from external disturbances may beobtained.

The invention contemplates also as an alternativeto the coaxial circuitof a high-frequency transmission medium comprising a circuit consistingof two conductors surrounded 'by a shield. Here, again, thehigh-frequency attenuation may be minimized by proper design withsubstantially gaseous insulation and the thickness of shield may bedeternnned so as to minimize interference from external disturbances.

Yet another form of transmission medium suitable for carrying out theinvention is a dielectric wave guide comprising a cylindriformdielectric material which may or may not be surrounded by a conductor.An advantageous form of such a transmission path is a hollow cylindricalconductor containing air or some other .gas as the dielectric. This typeof circult is particularly adapted to transmit waves of very highfrequencies, for example, of the order of a few centimeters or less inwave-length.

Using any of the above types of transmission media, the interconnectionof subscribers at will from the available channels involves a number ofnew and difiicult problems which are solved by the various features ofthe invention as hereinafter disclosed.

It is contemplated in the present invention that subscribers will beconnected together directly by means of the common medium, withoutnecessitating the interposition of a central office. Direct connectionbetween subscribers is eifected by a process of tuning or selection.This method takes advantage of the fact that the transmission medium iscommon to all of the subscribers, with all of the channels accessible toeach subscriber at will. The switching function is placed in the handsof the individual subscribers by eliminating the central ofiice in sofar as connections within the given area are concerned. Connection tosubscribers in other areas is carried out through the respective centralsubscriber.

offices which are joined by trunk channels oi circuits suitable for thepurpose.

One of the big functions involved in local exchange telephone systems isthat of switching so that each subscriber is connected with any other atthe central offices by the switching of mechanical contacts either as amanual operation or by automatic equipment under remote control of thesubscriber. In accordance with certain embodimentsof the invention, itis proposed to carry out this function of interconnecting subscriberswithout the switching of mechanical contacts. This is accomplished byhaving the subscribers permanently connected to the common medium andcompleting the electrical path directly by the subscriber at his ownpremises by frequency selection. Thus, an important object of theinvention is the provision of means whereby mechanical switching may bereplaced This operation is now carried out by electrical switching. Thiselectrical switching or interlinking is carried out by shifting thetuning of the circuits, by selecting the frequency of the carriercurrent, by shifting the frequencies in a modulator, or a combination ofthese. Selecting, modulating and multiplexing methods are, therefore,important features of the invention.

In general, the invention contemplates the assignment to each subscriberof a particular frequency which might preferably be related to histelephone number. The assignment of frequencies should be such as tofacilitate the interconnection of subscribers. g

The foregoing outline having indicated some of the principles of theinvention'as well as a few of its major objects, the complete inventionwith its various details, features and purposes will now be understoodfrom the following description when read in connection with theaccompanying drawings, in which Figure 1 shows a schematicrepresentation of the principal idea of the invention wherein a largenumber of stations are connected to a common transmission medium; Fig. 2shows an arrangement for a subscribers station whereby direct connectionmay be established with any other station; Fig. 3,

shows a method of channel selection that may be used with thearrangement of Fig. 2; Fig. 4 shows another such arrangement; Fig. 5shows an antenna arrangement which may be used with subscriber stationsof the type shown in Fig. 2; Fig. 6 is a schematic diagram indicatinghow connection may be established directly between subscribers, in thesame area, while connections between subscribers in different areas arecompleted through central offices over trunk circuits; Fig. '7 is aschematic diagram of a guiding or conducting network which is availablein common to a number of subscribers and a central station.

Referring to Fig. 1, there is illustrated one of the principal aspectsof the invention. In this figure is shown a common medim GM, to whichare connected a plurality of stations S1, S2; S3, etc., each of whichincludes transmitting, receiving and associated apparatus. Thisapparatus, which will be hereinafter described, is arranged to permitintercommunication between different pairs of stations over the commonmedium CM employing different frequency bands to permit simultaneouscommunication between different pairs of stations.

In Fig. 2 is shown a detailed circuit arrangee ment for a subscribersstation illustrating a possible form of the invention. In this case onlyone frequency band is required for each subscriber. Outgoingtransmission takes place on the called subscribers frequency andincoming transmission employs the calling subscribers frequency.

Before going into the detailed operation of the arrangement of Fig. 2,some of the elements forming a part of this arrangement may be brieflydescribed. The arrangement includes two rotary switching units SW1 andSW2 which are similar to units familiar in the art. The wipers of theseswitches may be stepped around by the stepping magnets MM1 and MlVIz inresponse to codes of impulses. The switches have the additional feature,however, that the position of the wipers may be controlled manually bymeans of the dials HCl and HCz. It will be understood that while theswitches as shown are designed to provide for a system having a maximumof 99 subscribers, the subscriber capacity may be readily increased bythe application of principles well known in the art.

Included also in the arrangement of Fig. 2 is a motor-driven toothedwheel TW, whereby the number assigned to the subscriber may betransmitted in the form of a series of interruptions of the transmittedcarrier frequency.

The operation of the arrangement of Fig. 2 in the placing of an outgoingcall will now be explained. By means of the controls HC1 and H02 thesubscriber first manually adjusts the switches SW1 and SW2 to a settingcorresponding to the number of the called party. The setting of the s1rows of these switches determines the response frequency of the variableselecting circuit VF1, while the setting of the sa rows determines thevalue of the variable condenser V01 which in turn controls the frequencyof the oscillator 0C. Thus the frequency of the oscillator 00 and theselecting circuit VF1 is adjusted to the receiving frequency which isassigned to the called subscriber.

Instead of manually controlling the positions of the switches SW1 andSW2 by means of I-IC1 and I-IC2, these switches may be adjusted by meansof the dial DI from any location remote from the switches. The contactsof the dial DI are normally open. The operation of the dial DI for thefirst digit of the called partys number transmits a number of pulsescorresponding to the digit dialed. These pulses are make pulses ratherthan interruptions of the circuit. The first pulse completes a circuitfrom ground, through the contacts a and c of the off-normal spring ONS,the relay SR and the stepping magnet MMr to battery. This operates thecontacts of switch SW1 one step. The off-normal springs ONS are operatedto the off-normal position and relay SR is operated. The second andsucceeding pulses of the first digit actuate MM1 by a new path throughcontacts b and c of the offnormal springs ONS and the make contact ofrelay SR. Relay SR being of the slow release type does not fall backduring the interruptions of the first digit.

After the first digit a pause allows relay SR to fall back. The pulsesof the second digit actuate stepping magnet MMz by completing a circuitfrom ground through contacts I) and c of the off-normal springs CNS andthe back contact of relay SR. The operation of these two banks ofswitches adjusts the selecting circuit VF1 and the frequency of theoscillator 00 to the called partys receiving frequency. Energizing ofrelay AR during the operation of MMz is prevented by the off-normalcontact ONC which is opened during the operation of the dial DI.

If, having adjusted the apparatus as described, the called party isbusy, his receiving carrier will be received by the antenna SA as wellas by his own antenna. The variable selecting circuit VF1 will pass thiscarrier frequency to the rectifier RFl. The output of the rectifier willenergize the relay RL1 whose winding is connected through a back contactof RLz. The operation of RL1 closes a circuit from battery through thebusy lamp BL to ground. (It will be understood that the separate sourcesof power supply shown in various places in Fig. 2 may, if desired, bereplaced by a single source.) The busy lamp indicates to the subscriberthat the called party is busy. He may then wait until the called partysfrequency becomes idle, or restore the switches to their normalpositions by pressing the release button RB. This energizes relay RL3and completes the circuit through the offnormal contacts in banks S3 ofswitches SW1 and SW2 and stepping magnets MM1 and MM2. RLg holds up longenough for SW1 and SW2 to reach their normal position.

When the called party is not busy, the relay RLl is deenergized and thebusy lamp is not lighted. The calling subscriber now takes his receiveroff the switchhook SH. This connects power supply to the oscillator OCand the modulator MO. The carrier from the oscillator 00 is rectified inthe rectifier RFz and energizes the relay RLz whose winding is connectedthrough a make contact of the switchhook and a back contact of R111. Theoperation of RLz accomplishes several results. First, it connectscarrier from the oscillator 00 to the modulator MO. Second, it disablesthe relay RLl so that the transmitted carrier does not aifect thisrelay. Third, it connects power supply to the motor DM which drives thetoothed wheel TW. The circuit for the power supply may be traced througha break contact of the slow-release relay RLs, a make contact of theswitchhook, and a make contact of RLz. The rotation of the toothed wheelTW serves to interrupt the carrier supply in accordance with the digitsof the number of the calling subscriber. These interruptions operateswitching units at the station of the called party which, in turn,adjust the called partys equipment so as to transmit 0n the frequency ofthe calling party as will be explained below.

After one revolution of the wheel TW, during which the complete code hasbeen dialed, an auxiliary lug on the dial closes a contact whichcompletes a circuit through the winding of the slow-release relay RL3.This relay RLs thereupon forms for itself a locking circuit through oneof its make contacts and a make contact of the switchhook. Thus therelay RL; remains operated until the calling subscriber hangs up. Theoperation of RL3 serves to interrupt the power supply to the motor DM sothat no further rotation of the wheel TW can occur. RLs also completes amake contact whereby the circuit of the transmitter STi is completed.Since the calling subscriber has adjusted his transmitting equipment tothe receiving frequency of the called party, and since the impulsestransmitted by the wheel TW have served to adjust the transmittingfrequency of the called party to the receiving frequency of the callingparty, conversation may now take place.

At the end of the conversation the calling party hangs up. At this timetwo circuits are closed to ground through a make contact of theslow-release relay RLs and a' break contact of the switchhook. One ofthese circuits leads through the wiper of the contact row $3 of theswitching unit SW1 to one of the ofi-normal contacts of this row andthence through a back contact of the stepping magnet Mlvh and thewinding of this magnet. The other path leads similarly through the wiperof rowss of switching unit SW2, an ofi-normal contact of this row, aback contact of the stepping magnet MMz and the winding of this magnet.These circuits cause the stepping magnets MM1 and MMz to buzz arounduntil each reaches its normal contact position. Thus the switching unitsare placed in readiness for the next connection.

In receiving a call from some other station in the system, thearrangement of Fig. 2 operates as follows: The calling station adjustsits transmitting frequency to the mid-band frequency of the fixedselecting circuit FF1. This frequency passes to the demodulator SD1which is continuously in operating condition. The rectified current inthe demodulator output passes through the blocking inductances BI andenergizes the relay FR. The operation of FR closes a circuit which pullsup the slow-release relay RR.

The dial at the distant partys station now interrupts the carrierfrequency in accordance with the number of the calling party. Thecircuit of relay FR will first be interrupted in accordance with thefirst digit of the number. The deenergizing of the relay FR will breakthe circuit of the relay RR, but since the latter is a slow-releaserelay it will continue to hold until the relay FR pulls up again.

When the armature of the relay FR falls back at the first break, itcloses a circuit through its break contact, a make contact of RR,contacts a and c of the off-normal springs ONS, the winding of theslow-release relay SR, and the stepping magnet MMI. Hence the relay'SRpulls up and at the same time the stepping magnet MMl operates toadvance the wipers of the switch SW1 by one step.

This circuit through the relay SR and the stepping magnet MM1 isimmediately broken by the reenergizing of the relay FR after the firstbreak impulse. The stepping magnet MM1 releases but since SR is aslow-release relay, it holds until the next impulse. The second breakimpulse again deenergizes FR and again closes a circuit through SR andMM1. This time, however, the circuit is completed over a new pathleading from ground over the back contact of FR, a make contact of RR,contacts 0 and b of the off-normal springs (closed when SW1 was advancedone step), the make contact of SR, the winding of SR, and the steppingmagnet MMi. Thus SW1 is advanced another step. Succeeding impulses serveto actuate Mh h just as the second impulse did.

After the last break impulse there will be a time interval before thedialing of the next digit, so that the slow-release relay SR will bedeenergized. This will break the circuit through this relay so that whenthe second set of impulses arrives, the stepping magnet MMi will be outof the circuit. The first break impulse of the second series causes therelay FR to fall back and thereby closes a circuit through the breakcontact of FR, the make contact of RR, off-normal contacts 0 and b, thebreak contact of SR, and thence in parallel through the winding of thestepping magnet MMz and the winding of the slow-release relay AR. Thestepping magnet MMz will be deenergized at the end'of the first breakimpulse, and the succeeding impulses will act each to advance theswitch'SW by one step just as the first one did. Consequently at theconclusion of the dialing the wipers of the switches SW1 and SW2 will bein positions which correspond to the number of the calling party. Theposition of these switches in turn sets the frequencies of' the variableselecting circuit VF1 and the oscillator 00 at the receiving frequencyof the calling subscriber.

The operation of the slow-release relay AR at the first impulse of thesecond digit energizes the relay BR whose winding is connected throughthe make contact of AR. Since AR is a slow-release relay, it will remainenergized throughout the pulses corresponding to the second digit. Atthe conclusion of these pulses, however, the operation cfFR will serveto deenergize AR, whereupon a circuit will be completed from the windingof the slow-release re-' lay CR, through the make contact of BR and thebreak contact of AR. Relay CR thereupon establishes for itself a newcircuit leading through a break contact of the switchhook to ground. Theoperation of CR closes the circuit of the subscribers bell SB which isconnected through a back contact of R114.

When the called subscriber takes the receiver off the switchhook toanswer, power supply is connected to the oscillator 00 and the modulatorMO as already described. The pulsing circuit is now disabled by theenergizing of an auxiliary winding of relay FR through a make contact ofthe switchhook. The operation of RLz connects carrier supply from theoscillator 0G to the modulator MO. Since CR is a slowrelease relay, itholds up for an instant after the operation of the switchhook. At thistime a circuit is completed through the winding of the slow-releaserelay RL4, a makecontact of CR, and a make contact of the switchhook,which operates RL4. Operation of R31 opens the bell circuit andenergizes RL3. RL3 locks itself up through one of its make contacts anda make contact of the switchhook SH. RL3 also cuts off the power supplyto the motor DM so that the dialing mechanism becomes inoperative andcompletes the circuit of the transmitter STl.

The equipment is now in readiness for conversation. The slow-releaserelay RLs remains operated during the conversation and is deenergizedwhen the receiver is replaced on the hook, but remains operated for aninterval. This closes two circuits through the back contact of theswitchhook SH, a make contact of RL3, the wipers in the 53 rows of SW1and SW2 and thence through the windings of MM1 and MMz so that theswitches are stepped around to their normal positions. When the variousredescribed in connection with Fig. 2. The con- 76,

densers connected to bank $1 of SW2 may have suitable values so that thefrequency interval determined by two successive condenser values of SW1is divided into ten parts. This makes a total of 99 frequency bandsavailable through combinations of the condensers in the .81 banks of SW1and SW2, and the inductance FI. It would be possible of course to obtainlarger numbers of frequency bands by adding other switches.

It Will be obvious to those skilled in the art that more complexvariable filter structures can be utilized by connecting other elementsin the circuit by means of additional banks of the switches SW1 and SW2.This is also included in the scope of the invention.

The banks of contacts s2 connect various values of capacity in thecircuit of the oscillator of Fig. 2, thereby adjusting its frequency ashas been previously explained. The operation of the rest of Fig. 3 hasbeen detailed in connection with Fig. 2.

Another possibility would be to derive the desired values of inductanceand capacitance for the variable filter VF1 of Fig. 2 from variablecondensers and inductances which might either be mounted directly on theshafts of the switching units or controlled by them. For this purposethe condensers and inductances to be controlled by the stepping magnetMlVh may be mounted on a common shaft which is provided with a wipertraversing an annular ring. This is shown in the perspective drawing ofFig. 4. The condensers and inductances to be controlled by the steppingmagnet MM2 are similarly shown mounted on another shaft. The condensersand inductances controlled by MM1 may be divided into stepsapproximately ten times those of the condensers and inductancescontrolled by MM2. The corresponding units on the two shafts may besuitably connected together and to the proper place in the variablefilter VF1.

Referring to Fig. 4 in response to the stepping magnet MM1 the shaft isstepped around to a position corresponding to the first digit of thedesired number thus adjusting the values of the inductance V11 and thecondenser VC1. The first 180 of rotation of the shaft covers the usefulrange of the circuit elements. Since these elements are continuouslyvariable any number of positions may be included in this range. Thenumber of teeth on the ratchet HA1 should therefore be twice the numberof positions employed. For ten digits the ratchet should have 20 teeth.The adjustment of V12 and VC2 is similarly controlled by MM2.

When the connection is taken down, a circuit is completed ..through thestepping magnet and its back contact, the wiper and annular ring, andthe make contact of RL 3 (Fig. 2) to ground. This completes therevolution of the shaft around to the starting point where an insulatedsegment breaks the circuit. The mechanism is now in readiness for thenext operation.

It will be noted that the arrangement of Fig. 4 eliminates all switchedcontacts in the selecting circuits. It will be seen in Fig. 2 that thecontacts which complete the circuit of the transmitter STi and that ofthe receiver SR might: be deleted, permanently closing the circuits ofthese elements, as shown by the dotted lines 'r--r and t--t in Fig. 2,without in any way interfering with the operation of the apparatus. Thiswould completely eliminate switched contacts from the circuit bet-weentwo subscribers. This is one of the features of the invention.

Radio transmission between subscribers who are provided with the stationarrangement of the type shown in Fig. 2 may be accomplished directly,using any suitable type of antenna. However, in order to obtainsubstantially lineof-sight transmission for the ultra-short range offrequencies, the transmission path may be carried over obstacles whichintervene between the antennas of individual subscribers by employing anarrangement of the type shown in Fig. 5.

Referring to this figure, the frequencies trans-- mitted by the varioussubscribers are carried through a central repeating station where theyare received on the antenna CA1 and re-radiated from the antenna CA2.These antennas CA1 and CA2 may be located upon a tall building or someother place having sufficient elevation. The antenna CA1 is designed toreceive from all subscribers antennas Within the assigned area while theantenna CA2 is of a type suitable for radiating to all subscribersantennas. Each subscribers antenna may be designed to radiate a beamtoward the antenna CA1 and to receive radiation from the antenna CA2.

In using the antenna system of Fig. 5 in conjunction with thesubscribers arrangement of Fig. 2, subscriber A, for example, might beassigned a receiving frequency f1 and subscriber D a receiving frequencyis. Subscriber A then would call subscriber D by setting histransmitting frequency at is, and the pulses transmitted from hisapparatus would adjust the transmitting frequency of subscriber D to thevalue f1.

As has been explained, the subscriber station arrangement of Fig. 2 isdesigned to provide a direct radio connection between the subscriberswithin a given area. It is proposed to interconnect such stations withsubscriber stations in other areas by providing in each area a centraloffice, so that a radio connection may be established between asubscriber and his central ofiice, using one of several frequenciesassigned for that purpose, and the connection completed over suitableinteroffice trunks. The method is illustrated schematically in Fig. 6.The central office apparatus to be used in carrying out this methodmight be patterned after that shown in Fig. 2. The types of trunks whichmay be used to interconnect the central offices are well known in theart.

In the arrangements which have been described for selecting channels,the entire selection has been accomplished at the channel frequency. Itwill be evident that this method of selection might be replaced by thewell-known superheterodyne method, with partial selectivity provided atthe channel frequency and the remaining selectivity furnished by asharp, intermediate frequency selecting circuit or filter.

It will be evident to those skilled in the art that arrangements forautomatically controlling the volume of the signals delivered by thesubscribers equipment may be incorporated in any of the subscribersstation arrangements which have been described, so that conversationbetween different subscribers may take place over a substantiallyconstant transmission equiva ent- It Will be noted that in thesubscriber station arrangement described above, the apparatus isdesigned to prevent a subscriber from listening in on the conversationsof other subscribers.

This, of course, is a very desirable feature for a telephone exchangesystem.

In the arrangements described it has been assumed that free space isemployed as the common transmitting medium. There will now be describedarrangements in which transmission over a high-frequency conducting orguiding path is employed instead of radio transmission. Such a pathwould take the form of a network extending to all subscribers within agiven area and to the central ofiice for that area.

Probably the simplest form of such a network would be one comprised ofordinary two-conductor circuits suitably interconnected and branched sothat the common circuit will be available at all desired points. Thecircuits comprising the network might, for example, consist of pairs ofopen-wire or cable conductors. A network of this kind is shownschematically in Fig. 7, where each line represents a pair ofconductors, C designates the central office and S a subscribers station.

It is contemplated also in accordance with the invention that the wirenetwork employed for distributing light and power currents might be usedas the common medium from which to derive high-frequency channels forthe telephone exchange system. The high-frequency channels would besuperposed upon the power network by carrier methods which are wellknown in the art.

Another form of transmission medium, and one which is peculiarlyadvantageous in that it is capable of transmitting a wide band offrequencies with comparatively low attenuation and which may be soshielded as to be practically immune to external disturbances, maybe'found in a circuit consisting of two conductors disposed coaxiallywith respect toone another. Such a circuit has been disclosed in thepatents to L. Espenschied and H. A. Aifel, No. 1,835,031, December 8,1931; H. A. Aifel and E. I. Green, No. 1,781,092, November 11, 1930,etc. In this form of circuit, low attenuation at high frequencies may beobtained by the employment of conductors whose high-frequency resistanceis suitably small and by the employment of a substantially gaseousdielectric. The outer conductor provides shielding against externaldisturbances, which shielding becomes more nearly perfect as thefrequency is increased, so that at high frequencies the noise due tothermal agitation in the conductors becomes the factor which determinesthe minimum transmission level.

As an alternative to the coaxial circuit, thehigh-frequency transmissionmedium might comprise a network of circuits, each consisting of twoparallel conductors surrounded by a shield. Circuits of this type aredisclosed in the applications of Green, Curtis and Mead, Serial No.674,- 762, Green and Curtis, Serial No. 674,763 and Green and Liebe,Serial No. 674,764, all filed June 7, 1933.

The invention contemplates also utilizing as the high-frequencytransmission medium a dielectric wave guide comprising a cylindriformdielectric material which may or may not be surrounded by a conductor.Such dielectric guides are disclosed in the patent applications of G. C.Southworth, Serial No. 661,154, filed March 16, 1933, and Serial No.701,711, filed Dev cember 9, 1933. An advantageous form of such a waveguide is a hollow cylindrical conductor containing air or some other gasas the dielectric. This type of circuit is particularly adapted totransmit waves of very high frequencies,

for example, of the order of a few centimeters or less in wavelength.

It is contemplated in accordance with the in: vention that thesubscriber set which has been described may be employed with any of theabove types of transmission paths. For this purpose, it is necessarymerely to substitute the transmission network for the radio path,replacing the connection of the apparatus to the radio antenna by aconnection suitable to the type of medium employed.

While the entire preceding discussion has been restricted to systemswherein a common transmission medium is employed for the provision oftelephone exchange service between subscribers, it will be understoodthat the invention contemplates the use of the same general methods forother types of communication. In particular, it will be seen that therehas been disclosed a medium which is capable of handling a wide band offrequencies from which a substantial number of television channels maybe derived.

It will furthermore be obvious that the general principles hereindisclosed may be embodied in many other organizations widely differentfrom those illustrated without departing from the spirit of theinvention as defined in the following claims.

What is claimed is:

1. In a telephone exchange system for establishing a plurality ofsimultaneous telephone connections between any one and any other of aplurality of stations taken in pairs over a common transmission medium,means for as signing to each station a fixed band on which it invariablyreceives, said band being different from those of other stations, meansat each station as a called station whereby it may be adjusted inresponse to a signal to transmit on the frequency band corresponding tothat, signal, and means at each calling station to send the signal forthe frequency band on which it is prepared to receive.

2. In a telephone exchange system, a plurality of stations, a commonmedium extending to each of the stations, means for assigning to eachstation a different carrier frequency at which it will receive, eachstation having a telephone number, the telephone numbers of the stationsbeing arranged in substantially the same order as their assignedreceiving frequencies, means at each one station for establishing atwo-way telephone connection between that one station and any otherstation by employing the frequencies assigned to the two stations fortransmission in respective directions, and manually controlled means ateach one station by which to actuate the said establishing means at bothstations.

3. A system for telephone communication between any one and any other ofa plurality of stations, said system including a common transmissionmedium extending to each of said stations, said medium being capable oftransmitting a plurality of frequency bands a different one of which isassigned to each station for reception, and means at each of saidstations for selecting at will any one of said frequency bands fortransmitting to another of said stations upon the band assigned to thedesired station, and means under control of said first station forautomatically adjusting said other station to transmit back on thefrequency band assigned to said first station.

4. A high frequency transmission system, in- 75 cluding a plurality ofstations, a common transmission medium, means at each of said stationsfor transmitting on any of a plurality of frequencies, each of saidstations being arranged to receive on a different one of saidfrequencies, and means for adjusting by said transmission thetransmitting frequency of the station receiving said transmission.

5. A static-n having a transmitter capable of sending on any of aplurality of frequencies and a receiver receiving on a fixed frequency,a variable filter having a range of adjustment extending to anyparticular one of said transmitting frequencies, and means responsive toan incoming frequency passed by said filter for disabling saidtransmitter. I

6. A station having a transmitter capable of transmitting on any of aplurality of carrier frequencies and a receiver receiving on a fixedfrequency, a variable tuned circuit having a range of adjustmentextending to any particular one of said transmitting frequencies, meansassociated with said tuned circuit and responsive to any incomingfrequency to which said circuit is tuned for preventing said transmitterfrom functioning, and means for indicating said condition.

7. A station having a transmitter capable of being adjusted andconnected for sending on any of a plurality of carrier frequencies and areceiver receiving on a fixed frequency, a common transmission mediumcapable of transmitting any of said transmitting frequencies, meansoperative at an intermediate stage of the complete adjustment andconnection of said transmitter for it to receive the carrier frequencyto which it is being adjusted, and means responding to a carrierfrequency so received to prevent said transmitter from sending on anycarrier frequency which is being transmitted over said medium, and meansfor indicating that said carrier frequency is being transmitted oversaid medium.

8. A subscribers station having a transmitter capable of sending on anyof a plurality of carrier frequencies, a receiver at said stationarranged to receive at only one carrier frequency, a second subscribersstation having a second receiver capable of receiving one of thefrequencies transmitted by said first transmitter, a second transmitterassociated with said second receiver, means for interrupting the carrierfrequency sent out by said first transmitter in a code, and meansresponsive to the receipt of said code by said second receiver foradjusting said second transmitter to transmit on a carrier frequencywhich said first receiver is capable of receiving.

9. A station having a receiver and a transmitter, said receiver beingcapable of receiving a single fixed carrier frequency and beingunresponsive at all times to any other frequency, said transmitter beingcapable of operating on any of a plurality of carrier frequencies, meansresponsive to signals received by said station upon said fixed carrierfor adjusting the frequency on which said transmitter operates.

10. In a telephone exchange system, a plurality of subscribers stations,a common transmission medium extending to each of said stations, each ofsaid stations being continuously in readiness to receive transmissionfrom said medium at a unique carrier frequency, means for transmittingfrom any one of said stations on any of said receiving frequencies,means at the called station for signaling the subscriber, means undercontrol of the calling station for adjusting the transmitting frequencyof the called station to the receiving frequency of the calling station,and means enabling two-way conversation between said calling and calledsubscribers.

11. A station capable of receiving signals on only a single fixedfrequency, said station having a transmitter capable of operating on anyof a plurality of frequencies, means for locally adjusting the operationof said transmitter to any particular frequency, means responsive tosignals received by said station for adjusting the operation of saidtransmitter to any particular frequency, and means to prevent one ofsaid adjusting means from interfering with the operation of the other.

12. A station capable of receiving signals on only a single fixedfrequency, said station comprising a transmitter capable of operating onany of a plurality of frequencies, means for locally adjusting theoperation of said transmitter to anyparticular frequency, meansresponsive to signals received by said station for adjusting saidtransmitter from some remote point to operate on a particular frequency,and means for preventing said remote adjusting means from interferingwith the operation of said local adjusting means.

13. A telephone exchange system, including a plurality of subscribersstations each arranged to receive only at a fixed carrier frequency,this frequency being different for different stations, a commontransmission medium extending to each of said subscribers stations,means whereby a two-way telephone connection may be established oversaid medium directly between any one and any other of said stationswithout passing through a central switching office, and manual means ateach single station to actuate said establishing means at both stations.

14. A telephone exchange system, including a plurality of subscribersstations each arranged to receive only at a fixed carrierfrequency,-this frequency being different for different stations, acommon transmission medium extending to each of said subscribersstations, means whereby a twoway telephone connection may be establishedover said medium directly between any one and any other of said stationswithout passing through a central switching ofiice, manual means at eachsingle station to actuate said establishing means at both stations, andmeans whereby a plurality of such connections may be established andsimultaneously maintained between different pairs of stations.

15. A system for telephone communication between any one and any otherof a plurality of subscribers stations, including means at eachsubscribers station for transmitting on any of a plurality offrequencies and receiving only on a single different fixed frequency,said receiving frequency being different for each subscribers station,means at each subscribers station for establishing direct two-waycommunication with any other of said stations, and manual means at eachsingle station to actuate said establishing means at both stations.

16. A system for telephone communication between any one and any otherof a plurality of subscribers stations, including means at eachsubscribers station for transmitting on any of a plurality offrequencies and receiving only on a single different fixed frequency,said receiving frequency being different for each subscribers station,means at each subscribers station for establishing direct two-waycommunication with any other of said stations, manual means at eachsingle station to actuate said establishing means at both stations, andmeans whereby a plurality of such communications may be carried onsimultaneously between different pairs of stations.

17. A system for the exchange of intelligence between any one and anyother of a plurality of subscribers stations, including an adjustabletransmitting circuit and a fixed receiving circuit both at eachsubscribers station, means at each station to adjust the adjustabletransmitting circuits at that station and selectively at another stationfor direct communication therewith, and means to notify any station ofsaid system that any desired frequency is in use.

18. A system for the exchange of intelligence between any one and anyother of a plurality of subscribers stations, including an adjustabletransmitting circuit and a fixed receiving cirouit both at eachsubscribers station, means at each station to adjust the ajdustabletransmitting circuits at that station and selectively at another stationfor direct communication therewith, and means whereby a plurality ofsuch communications may be carried on simultaneously between differentpairs of stations at different frequencies.

19. A system for the two-way exchange of intelligence between any oneand any other of a plurality of subscribers stations, each subscribersstation having assigned thereto for communication of intelligence in onedirection a fixed frequency which is different from that of othersubscribers stations and being adapted to use a variable frequency forcommunication of intelligence in the opposite direction, eachsubscribers station having means whereby when it is a calling station itmay be adjusted to utilize the fixed frequency assigned to the calledsubscribers station, each subscribers station also having means wherebywhen it is a calling station it may control the adjustment of the calledsubscribers station to utilize the fixed frequency assigned to thecalling subscriber.

20. A system for the exchange of intelligence between any one and anyother of a plurality of subscribers stations, each subscribers stationhaving assigned thereto for receiving purposes a fixed frequency whichis different from that of every other statiorr and being adapted to usea variable frequency for transmitting, each subscribers station havingmeans whereby when it is a calling station it may be adjusted to utilizethe receiving frequency assigned to the called station, each subscribersstation also having means whereby when it is a calling station it maycontrol the adjustment of the called subscribers station to utilize thereceiving frequency assigned to the calling subscribers station.

21. A system for telephone communication between any one and any otherof a plurality of subscribers stations, said system comprising a commontransmission medium extending to each of said subscribers stations,means whereby said transmission medium may be utilized simultaneouslyfor a plurality of telephone connections directly between any one andany other of said subscribers stations in pairs over different frequencybands, said means comprising a selective circuit of fixed frequency forreceiving at each station and a selective circuit of adjustablefrequency for transmitting at each station, means at eachrone station toadjust the circuits of the adjustable frequency at that one station andalso at one other station selectively, and means to notify anysubscribers station of said system that a desired frequency band is inuse.

22. A telephone exchange system, comprising a plurality of radiotelephone stations each capable of transmitting on any of a plurality ofcarrier frequencies and receiving on a fixed carrier frequency difierentfor each station, means whereby a plurality of simultaneous two-waytelephone communications may be effected directly between any one andany other of said stations in pairs on the fixed carrier frequencies forreceiving each way with adjustment thereto of the correspondingtransmitting frequencies, and means to notify any station of said systemthat any desired carrier frequency is in use.

23. The method of attuning two of a large number of numbered stationsfor signaling between them which consists in making code impulses at onestation according to the call number of that one station, and therebyadjusting apparatus at the other station to the same frequency asapparatus at the said one station.

24. In a system of numbered signaling stations each adapted to operateat a frequency value in simple relation to its call number, the methodof bringing one station to the same frequency as another which consistsin making code impulses at one of two stations involved according to thecall number of that one of the two stations, and thereby adjusting thefrequency at the other station to a value corresponding to that number.

25. In combination, means to make code impulses at one stationcorresponding to the call number of said one station of a plurality ofmore than two stations, and means controlled thereby to adjust thefrequency of apparatus at another station selectively to match thefrequency of apparatus at the said one station.

26. The method of selectively attuning two stations at one of them fortwo-way communication which consists in making code impulses at the onestation according to the call number of the other station and therebyadjusting the frequency of transmitting apparatus at the one station tomatch the frequency of receiving apparatus at the other station, andsending code impulses of the one station to the other station andthereby adjusting the frequency of transmitting apparatus at the otherstation to match the frequency of receiving apparatus at the onestation.

27. Transmitting apparatus and receiving apparatus at each of aplurality of more than two signaling stations, one apparatus at eachstation being adjustable as to frequency and the other employing a fixedfrequency, means at each station as a calling station to adjust thefrequency of its adjustable apparatus to match the frequency of thefixed frequency apparatus of any one other station as a called station,and means also at such calling station to adjust the frequency of theadjustable apparatus of the called station to match the frequency of thefixed frequency apparatus of the calling station.

WARREN H. TIDD.

